Earth-like planet may be discovered in 2013

According to a popular news item that has been making the rounds recently, some astronomers are predicting that the first "Earth" beyond our solar system will be discovered during this year.

By "Earth," we mean a planet, rocky in nature (as opposed to gaseous, like Jupiter), about the same size and mass as Earth, orbiting its parent sun in what we call the "habitable zone," i.e., at a distance where temperatures are such that water is able to exist in its liquid state. Such a planet would be capable, theoretically at least, of supporting some kind of life as we know it.

Of the more than 850 planets beyond our solar system that have been discovered so far, we have not quite found an "Earth," although some worlds are pretty close.

We've found worlds that are about the same size as Earth, but orbiting closer to their parent sun and thus significantly hotter than Earth. The recently reported planet orbiting one of the stars in the Alpha Centauri system is an example.

We have also found planets that are orbiting within their respective stars' habitable zones, but these are quite a bit larger than Earth, and some of these appear to be large gaseous planets like Jupiter. Some of these worlds could, conceivably, possess habitable moons -- such as the fictional "Pandora" in the recent science fiction movie Avatar -- but this can only be speculation for now.

Only the next 12 months will tell whether or not the aforementioned prediction comes true.

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In order to evaluate its likelihood, it is worthwhile to examine the methods by which planets are detected in the first place. Although these methods have been known and written about for many decades, it has only been within the past 15 to 20 years that our technology has progressed to the point where valid search efforts became possible.

In theory, planets can be directly detected, i.e., we can detect the planets themselves via the light they reflect from their parent sun. In practice, however, this is enormously difficult since the planets would be extremely dim objects located close to their much brighter sun.

Only a very small handful of the planets discovered thus far have been detected in this fashion, usually with instruments like the Hubble Space Telescope; a planet about twice the size of Jupiter orbiting the bright star Fomalhaut (now high in our southern sky during the evening hours) at about three times Pluto's distance from our sun is one of the few known examples.

The overwhelming majority of planets that have been discovered thus far have been "indirectly" detected, i.e., we have detected them via the effects they have upon their parent sun.

Strictly speaking, it is not correct to say that the planets in our solar system orbit the sun; rather, the planets and the sun orbit around the entire system's center of gravity.

While the motion of the sun around this is very small, with sensitive enough equipment this can be detected, particularly if changes in the sun's spectrum caused by the Doppler effect (i.e., motion towards and away from the observer) are being looked for. This is precisely how the majority of the planets discovered so far have been found.

Large planets, and planets orbiting very close to their parent stars, would be the easiest to detect via this method, and most of the early discoveries -- including the first such discovery around the star 51 Pegasi near the "Great Square of Pegasus" now in the western sky during the evening hours -- were these so-called "hot Jupiters."

As our instrumentation has improved over the years, we have detected both smaller planets and planets farther away from their respective parent suns. The recent Alpha Centauri planet was found in this manner.

If a planet's orbit around its parent sun is oriented such that it is almost edge-on to our line of sight, it will periodically cross in front of its sun, causing small but regular drops in that star's brightness. This is usually referred to as the "transit" method, analogous to events like the transit of Venus across the sun that took place last summer. Since only 1 percent to 2 percent of stars would be expected to have planets in this type of configuration, this method is impractical to apply to individual stars.

But if we examine a very large sample of stars, we might expect to detect a reasonable number of planets. This is precisely the rationale for the Kepler mission, which was launched in 2009 and which is continuously monitoring the brightness of tens of thousands of stars located near the western "wing" of the constellation Cygnus the swan, now located in our northwestern sky during the evening hours.

As of now, Kepler has discovered more than 100 confirmed planets, with more than 2,000 additional candidate planets awaiting confirmation, and as many as 18,000 possible detections. The transit method is especially useful in finding Earth-sized worlds, and Kepler has indeed found most of the "almost Earth" planets that have been reported thus far.

If an "Earth" is indeed discovered during 2013, it will likely be a Kepler detection.

When a planet transits across its sun, part of its sun's light will pass through the planet's atmosphere. In theory, the spectrum of the star's light can be examined to see what gases might be present in that atmosphere. This has already been accomplished for a handful of "hot Jupiter" transiting planets.

Since oxygen is an unstable gas that bonds rapidly with other substances, if we detected a substantial oxygen content in an Earth-sized planet orbiting within its star's habitable zone, then we will have almost certainly detected another "Earth" -- and perhaps this is indeed only a matter of time.

Alan Hale is a professional astronomer who resides in Cloudcroft. He is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His website is www.earthriseinstitute.org.